Suppose your opponent has a Planeswalker X. Do you decided where the redirection goes to?
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No, Sickening Dreams can't deal damage to a planeswalker (unless that planeswalker is also a creature), and you can't choose to have Sickening Dreams deal damage to a planeswalker instead of to a player, without more (review C.R. 306.7).
In general, the update for Dominaria, which eliminated the planeswalker redirection effect, didn't change any "each opponent" or "each player" in the text of spells or abilities (examples include Sickening Dreams, Atarka's Command, Earthquake, Bloodfire Colossus, and Flame Rift). See also this thread and this thread.
If it was changed from "Each Player" to "Target Player", would this be allowed?
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If it was changed from "Each Player" to "Target Player", would this be allowed?
That change alone wouldn't suffice. In any case, use the Oracle text of a card to determine its wording (C.R. 108.1). In general, if the Oracle text of a spell or ability says "target ... planeswalker" or "any target" (C.R. 115.4), the spell or ability can target a planeswalker. However, a spell or ability that targets only a "player" can't target a planeswalker, since planeswalkers are not players (C.R. 115.1, 109.1, 110.4, 109.2). Also, in general, the update for Dominaria changed the text of older spells and abilities that could cause damage to "target player" or "target creature or player" so they could target a planeswalker. But again, check the Oracle text of the card in question (see, for example, Firesong and Sunspeaker, which was first released in Dominaria).
In particular, the Dominaria update didn't change the text of Sickening Dreams, which still says "Sickening Dreams deals X damage to each creature and each player", not "...and target player" or "...to each creature, each player, and each planeswalker" (C.R. 108.1). See also this thread.
Can Sickening Dreams hit Planeswalkers?
Suppose your opponent has a Planeswalker X. Do you decided where the redirection goes to?
Vintage Cube Cards Explained
Here are some other articles I've written about fine tuning your cube:
1. Minimum Archetype Support
2. Improving Green Archetypes
3. Improving White Archetypes
4. Matchup Analysis
5. Cube Combos (Work in Progress)
Draft my Cube - https://cubecobra.com/cube/overview/d8i
In general, the update for Dominaria, which eliminated the planeswalker redirection effect, didn't change any "each opponent" or "each player" in the text of spells or abilities (examples include Sickening Dreams, Atarka's Command, Earthquake, Bloodfire Colossus, and Flame Rift). See also this thread and this thread.
If it was changed from "Each Player" to "Target Player", would this be allowed?
Vintage Cube Cards Explained
Here are some other articles I've written about fine tuning your cube:
1. Minimum Archetype Support
2. Improving Green Archetypes
3. Improving White Archetypes
4. Matchup Analysis
5. Cube Combos (Work in Progress)
Draft my Cube - https://cubecobra.com/cube/overview/d8i
In particular, the Dominaria update didn't change the text of Sickening Dreams, which still says "Sickening Dreams deals X damage to each creature and each player", not "...and target player" or "...to each creature, each player, and each planeswalker" (C.R. 108.1). See also this thread.
EDIT (Feb. 23; Feb. 25): Edited.
EDIT (Mar. 4): Correctness edit.